It seems to be related to saving settings via the cloud instead of the available non-tracking cookie.

so if you are saving using ddg cloud settings you are more likely to see what you are seeing so try saving to cookie as well or you can try using what they call a bookmarklet instead which would be https://duckduckgo.com/?kae=d&kak=-1

i have also seen what you are seeing on slow connections - seems that the default theme is painted then the dark theme is overlayed which creates the effect you are seeing - it is pretty easy to ignore